Progress in Physical Geography

Transcription

Compared to the whitewater streams that tumble down mountainsides, the meandering rivers of the plains may seem tame and lazy. But mountain streams are corralled by the steep-walled valleys they carve – their courses are literally set in stone. Out on the open plains, those stony walls give way to soft soil, allowing rivers much more freedom to shift their banks and set their own ever-changing courses to the sea: courses that almost never run straight. At least not for long, because all it takes to turn a straight stretch of river into a bendy one is a little disturbance and a lot of time  – and in nature, there’s plenty of both.       Say, for example, that a muskrat  burrows herself a den in one bank of a stream. Her tunnels make for a cozy home,  but they also weaken the bank, which eventually begins to crumble and slump into the stream. Water rushes into the newly-formed hollow, sweeping away loose dirt and making the hollow even hollower, which lets the water rush a little faster and sweep away a little more dirt from the bank...and so on, and so on . As more of the stream’s flow is diverted into the deepening hole on one bank and away from the other side of the channel, the flow there weakens and slows. And since slow-moving water can’t carry the sand-sized particles that fast-moving water can, that dirt drops to the bottom and builds up to make the water there shallower and slower, and then keeps accumulating until the edge of the stream becomes new land on the inside bank.         Meanwhile, the fast-moving water near the outside bank sweeps out of the curve with enough momentum to carry it across the channel and slam it into the other side, where it starts to carve another curve . And then another, and then another, and then another. The wider the stream, the longer it takes the slingshotting current to reach the other side, and the greater the downstream distance to the next curve. In fact, measurements of meandering streams all over the world reveal a strikingly regular pattern : the length of one S-shaped meander tends to be about six times the width of the channel . So little tiny meandering streams tend to look just like miniature versions of their bigger relatives.      As long as nothing gets in the way of a river’s meandering , its curves will continue to grow curvier and curvier until they loop around and bumble into themselves. When that happens, the river follows the straighter path downhill, leaving behind a crescent-shaped remnant called an oxbow lake. Or a billabong. Or a lago en herradura. Or a bras mort ...      We have lots of names for these lakes, since they can occur pretty much anywhere liquid flows – which brings up an interesting question: what do the Martians call them?